He was moved into intensive care on Monday night. “Over the course of this afternoon, the condition of the Prime Minister has worsened and, on the advice of his medical team, he has been moved to the Intensive Care Unit at the hospital,” a spokesman said then.

Further details were not released by his office.

Aides said then that he was still conscious, but Foreign Secretary Dominic Raab was placed in charge by Johnson “where necessary,” a spokesman said.

It was a seemingly quick deterioration after Johnson had tweeted earlier Monday that he was in “good spirits and keeping in touch with my team.”

Peter Summers/Getty U.K. Prime Minister Boris Johnson

A spokesperson previously told PEOPLE that Johnson’s symptoms included “a temperature and a cough” but described claims on a Russian news site that he was on a ventilator as “disinformation.”

Johnson tweeted earlier Monday that he’d gone into the hospital on Sunday night “on the advice of my doctor.”

“I went into hospital for some routine tests as I’m still experiencing coronavirus symptoms,” he wrote then.

In another post, he praised the country’s medical staff. “I’d like to say thank you to all the brilliant NHS staff taking care of me and others in this difficult time. You are the best of Britain. Stay safe everyone, and please remember to stay at home to protect the NHS and save lives.”

The prime minister is receiving treatment at St. Thomas’ hospital in central London, which stands directly opposite the Houses of Parliament and is little more than a five-minute stroll from the Downing Street home he shares with 32-year-old fiancée Carrie Symonds.

Symonds, who is pregnant with the couple’s first child, is also struggling with the symptoms of COVID-19.

On Saturday, she tweeted that she’d spent the past week in bed: “I haven’t needed to be tested and, after seven days of rest, I feel stronger and I’m on the mend.”

As of Tuesday, there had been more than 52,000 confirmed cases of the coronavirus in the U.K. and 5,300 deaths, according to available data.

As information about the coronavirus pandemic rapidly changes, PEOPLE is committed to providing the most recent data in our coverage. Some of the information in this story may have changed after publication. For the latest on COVID-19, readers are encouraged to use online resources from CDC, WHO, and local public health departments. To help provide doctors and nurses on the front lines with life-saving medical resources, donate to Direct Relief here.